Abstract
A fatigue-resistant Nitinol instrument has a working portion in the deformed monoclinic martensitic state and an austenite finish temperature in the range of 40 to 60 C. Because the operating environment of the instrument is about 37 C., the working portion remains in the monoclinic martensitic state during its use. The relatively high austenite finish temperature and fatigue resistance is achieved by subjecting the nickel-titanium alloy to a final thermal heat treat in a temperature range of about 410 to 440 C. while the nickel-titanium alloy is under constant strain of about 3 to 15 kg. Further, the high austenite finish temperature is achieved without subjecting the alloy to thermal cycling to produce shape memory. Additionally, there are no intermediate processing steps occurring between obtaining a finished diameter of the wire or blank through cold working and the final thermal heat treat under constant strain.
Claims
1. An endodontic instrument comprising: a working portion composed of a nickel-titanium alloy and configured for use in the human body, the working portion having cutting surfaces, an austenite finish temperature in the range of 40 to 60 C., and a shape memory restoration such that, after a 90 bend and release, the working portion when released returns less than 88; the working portion having been subjected to a first heat treat while under constant strain before the cutting surfaces are formed and subjected to a second heat treat after the cutting surfaces have been formed.
2. An endodontic instrument according to claim 1 further comprising the first heat treat is at a different temperature than the second heat treat.
3. An endodontic instrument according to claim 1 wherein the first heat treat is in a temperature range of 410 to 440 C.
4. An endodontic instrument according to claim 1 wherein the second heat treat is in a range of 248 to 500 C.
5. An endodontic instrument according to claim 4 wherein the second heat treat is in a range of 300 to 450 C.
6. An endodontic instrument according to claim 1 wherein the working portion when released returns no more than 75.
7. An endodontic instrument according to claim 6 wherein the working portion returns to no more than 60.
8. A method of manufacturing an endodontic instrument from a nickel-titanium blank to obtain improved fatigue failure characteristics, the method comprising the steps of: (i) applying an elongational strain deformation to the blank within its reversible strain limit at a first heat treat temperature; (ii) after step (i), forming an instrument from the blank by performing machining operations; and (iii) after step (ii), heat treating the instrument at a second heat treat temperature.
9. A method according to claim 8 further comprising the first heat treat is at a different temperature than the second heat treat.
10. A method instrument according to claim 9 wherein the first heat treat is in a temperature range of 410 to 440 C.
11. A method instrument according to claim 9 wherein the second heat treat is in a range of 248 to 500 C.
12. A method according to claim 11 wherein the second heat treat is in a range of 300 to 450 C.
13. An method according to claim 8 wherein after step (iii) the endodontic instrument has an austenite finish temperature in a range of 40 to 60 C.
14. A method according to claim 8 wherein after step (iii) the endodontic instrument has a shape memory restoration such that, after a 90 bend and release, the working portion when released returns less than 88.
15. An endodontic instrument according to claim 14 wherein the working portion when released returns no more than 75.
16. An endodontic instrument according to claim 15 wherein the working portion returns to no more than 60.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) FIG. 1 is an elevational view of an endodontic instrument or file. The endodontic file is one example of a medical device that can be successfully manufactured by employing Nitinol material which is made according to the present invention.
(2) FIG. 2 is an elevational cross-sectional view of a human molar, its root canal system and the coronal area penetrated by a hole to expose the root canal system. The endodontic file of FIG. 1, when positioned within the root canal, is subjected to substantial bending and torsional stress as it cleans and shapes the root canal. A Nitinol material made according to the present invention significantly increases the file's resistance to cyclic fatigue.
(3) FIG. 3 illustrates prior art cold working procedures that can be employed in preparing Nitinol wire for the prior art final manufacturing steps as shown in FIG. 4 or the steps of the present invention as shown in FIG. 8.
(4) FIG. 4 is a diagrammatic illustration of steps employed in a prior art process to treat the Nitinol wire of FIG. 3 so that it can be employed for producing instruments having greatly improved resistance to cyclic fatigue. The present invention eliminates the need for the steps of FIG. 4.
(5) FIG. 5 is a graph illustrating the hysteresis effect as Nitinol is transitioned between martensite and austenite phases.
(6) FIG. 6 illustrates diagrammatically the transitions of Nitinol between austenite and martensite phases in response to changes in temperature and deformation.
(7) FIG. 7 shows diagrammatically the changes in phases of Nitinol in response to changes in temperature and stress as is shown in FIG. 6, but in somewhat greater detail, as the Nitinol is subjected to the prior art process of FIG. 4.
(8) FIG. 8 is a diagrammatic illustration of steps employed in the present invention. These steps may follow the cold working steps illustrated in FIG. 3. The steps of FIG. 8 eliminate the need for processing steps similar to or the same as those illustrated in FIG. 4 or those required to produce shape memory properties. After machining operations, the finished instrument may be subjected to a finished instrument heat treat to counteract any degradation that may have occurred to fatigue performance due to machining and restore and enhance the fatigue characteristics achieved by the process of FIG. 8.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
(9) The starting material for use in an apparatus and method practiced according to this invention is a Nitinol composition consisting of preferably 55.8+/1.5 wt. % nickel (Ni), with the balance being that of titanium (Ti). There are also trace elements including iron (Fe), chromium (Cr), copper (Cu), cobalt (Co), oxygen (O), hydrogen (H), and/or carbon (C), generally less than 1 wt. % each. The Nitinol composition may also consist of 50+/10 wt. % nickel, with the balance being that of titanium and the trace elements.
(10) Referring first to FIG. 3, a prior-art process shows the steps normally employed to convert a Nitinol wire into a stable martensite state useable for manufacturing fatigue resistant devices. One example of this type of device is the endodontic instrument illustrated in FIGS. 1 and 2. In the prior art process, untreated Nitinol wire is unwound from a spool and passed through an annealing oven, a series of dies, and final heat thermal processing operation. The temperatures used and the amount of cold working depend upon such factors as the amount of stiffness (or, conversely, flexibility) desired in the wire and the diameter of the wire. Typically, the cold working step achieves about a 45+/10% reduction in cross-sectional area of the wire and is followed by final heat thermal processing at about 500 to 800 C. (930 to 1475 F.) for a time period sufficient to anneal the wire based upon its diameter. The process according to the present invention may be preceded by these cold working steps but eliminates the need for the high temperature anneal of FIG. 3.
(11) Another prior art processdisclosed in U.S. Pat. No. 7,648,599subjects the Nitinol wire from FIG. 3 to a micro-twining and de-twinning process (also known as a training process). FIG. 4 shows the steps of the training process. The Nitinol wire is placed under a constant 1 to 10% elongation or strain as it is repeatedly thermal cycled between a cold and hot bath. The cold bath is at a temperature in the range of about 0 to 10 C. (32 to 50 F.) and the hot bath is in the range of about 100 to 180 C. (212 to 356 F.). As a result, the micro-twinning structure undergoes a de-twinning or realignment process to achieve an energetically stable martensitic structure with reduced interfacial friction and residual deformation. This contributes to the improved fatigue resistance of the Nitinol wire which results from this process.
(12) Nitinol as an alloy exists in two naturally occurring forms, that is, in the austenite form and in the martensite form. The alloy transitions between martensite and austenite, and can stabilize as either martensite or austenite, within a given temperature range (see FIG. 5). The austenite phase in the alloy is plotted as a function of the temperature, with several important transition temperatures marked. A.sub.S indicates the temperature at which the austenite starts and A.sub.F indicates the temperature at which the alloy is 100% in the austenite phase. M.sub.S and M.sub.F indicate the martensite start and finish temperatures, respectively. Starting with Nitinol in its martensitic state at temperature M.sub.F and increasing the temperature above M.sub.F to A.sub.S, the austenite form begins. As the temperature continues to increase above A.sub.S, the austenite form increases as a percent of the alloy until the temperature reaches A.sub.F. The alloy will remain in the 100% austenite form at temperatures at or above A.sub.F.
(13) Note the austenite and martensite phase transformations do not occur at the same temperature. Rather, a hysteresis loop exists corresponding to the phase transformation. In addition, a M.sub.D temperature exists. The M.sub.D temperature is the highest temperature at which strain-induced martensite can exist, i.e., the temperature above which martensite cannot be induced by strain. (The terms, strain, elongation, tension, stress and deformation are used interchangeably in this context.) Nitinol in the martensite form can demonstrate significantly improved fatigue resistance relative to Nitinol in the austenite form.
(14) As would be understood by those skilled in the art, the specific temperatures at which Nitinol transitions occur are very sensitive to small variations in the alloy's content of nickel, titanium and any other trace elements. Therefore, Nitinol's properties can be tailored for specific applications by controlling the alloy's composition.
(15) FIG. 6 pictorially illustrates the transition of Nitinol between the austenite form and the martensite form. The nickel is illustrated by the large non-shaded circles. The titanium is illustrated by the small shaded circles. When the alloy is in the austenite state, the arrangement of atoms is orderly. As the temperature of the alloy cools, the atomic structure changes from the initial orderly structure to a twinned martensite arrangement or state. In this twinned martensite state, and without a significant change in temperature, the alloy can be subjected to strain in order to transform into deformed martensite or de-twinned martensite state. The alloy can remain in this state until heat is applied to reach the level of the austenite start (A.sub.S). As further heat is applied the alloy returns to the 100% austenite state (A.sub.F).
(16) The shape memory and superelastic properties of Nitinol may be understood in terms of the phase transformations the alloy undergoes under various conditions. Shape memory refers to the ability to restore an originally memorized shape of a deformed Nitinol sample by heating it. Referring to FIG. 7, the alloy is heated to a temperature above A.sub.F and, when in the austenite state, formed into a desired shape. This causes the alloy to memorize the desired shape. Lowering the temperature below M.sub.S, moves the alloy into the twinned martensite state. If a deformation-inducing strain is applied to the alloy when in this state, the alloy will move to the deformed martensite state and will retain that shape even after the strain has been removed. Then, if the alloy is again heated to a temperature above A.sub.F, a thermo-elastic phase transformation takes place. The element returns to its previously memorized austenite shape, thereby regaining its strength and rigidity.
(17) As mentioned previously, the present invention does not require the high temperature, final thermal heat processing of FIG. 3 (after cold working), nor does it require the training process of FIG. 4, in order to achieve a martensitic structure that produces superior and unexpected fatigue performance. Unlike these prior art processes, the Nitinol material here is subjected to final thermal heat treat at about 410 to 440 C. (770 to 824 F.) while being subjected to a preferably constant strain of in the range of about 3 to 15 kg (see FIG. 8).
(18) In a preferred embodiment, wire 15 that has been cold worked (see FIG. 3) is passed through an oven 30 for final thermal heat treat while under strain (FIG. 8). Wire 15 going out of the cold working process may be passed directly to oven 30 or first wound about a spool 20. Wire 15 wound about spool 20 could also be supplied by a manufacturer of Nitinol alloy products and already cold worked to the desired dimension for subsequent machining operations. The treated wire 25 may be wound about a finish spool 30. Wire 25 is in condition to be used to manufacture products, specifically instrumentation or other products that require a high degree of flexibility combined with an unusually high fatigue resistance characteristic and high austenite finish temperature.
(19) The resultant Nitinol material or wire 25 is in a transitory martensitic state (i.e. in the deformed monoclinic state) and has the following properties:
(20) TABLE-US-00001 Properties Austenitic finish temperature A.sub.F ( C.) ~30-60 Ultimate tensile strength >200 Ksi Permanent set (PS @ 7 Mpa) 20 Ultimate tensile strength to upper ~2.5 plateau stress ratio
A person of ordinary skill in the art understands that the A.sub.F is very sensitive to changes in nickel composition and, therefore, to hold the A.sub.F at a desired level, the nickel composition must be held steady for any manufacturing process to achieve a repeatable A.sub.F. For example, the estimated percentage of nickel to accomplish an A.sub.F of about 50 C. (122 F.) using the process of FIG. 8 would be approximately 55.3%. Therefore, the percentage of nickel used to achieve a desired A.sub.F by way of the above method may be altered as appropriate.
(21) Endodontic instruments (see FIGS. 1 & 2) made using Nitinol wire which was prepared according to this method (FIG. 8) were tested at room temperature in a rotary system along a 90 curve. The instruments averaged about 11 minutes to failure. Instruments made according to the prior art training process of FIG. 4 averaged about 2.3 minutes to failure (still far above that of instruments made only according to the process of FIG. 3).
(22) The instruments also exhibited reduced shape memory. Shape memory is not an asset for any instrument which must traverse a curve or be positioned within a curve because shape memory is a restoring force. To test the shape memory properties of an instrument made according to the process of FIG. 8, the instrument was placed in a fixture and bent at a 90 angle and released. This bend and release was repeated two more times. An instrument without any shape memory will remain bent at 90 once released. Typically, a Nitinol instrument with shape memory properties exhibits about 88 of restoration. That is, after being bent 90, the instrument returns to a position about 2 less than its original starting position. In other words, instead of traveling a full 180 (90 bend, 90 return), the instrument travels about 178 total.
(23) A Nitinol instrument made according to the process of FIG. 4 is typically in the range of about 10 to 15 off of straight after the repeated bend-and-release test (exhibiting roughly 165 to 170 of total travel). The shape memory restorative property is in the range of about 80 to 90 percent.
(24) Instruments made according to FIG. 8 exhibited even less shape memory than those made according to FIG. 4, being about 25 to 35 off of straight after repeated bend-and-release (roughly 145 to 155 of total travel). Therefore, the shape memory restorative property of the instrument is about 60 to 75 percent.
(25) The medical or dental instrument provided by the method of FIG. 8 (and by the method of FIGS. 3 and 4) typically undergoes subsequent machining operations to produce the desired final instrument. For example, grinding operations may produce the helical flutes and cutting edges of the endodontic instruments of FIGS. 1 and 2. Because of mechanical deformation and frictional heat caused by the grinding, the fatigue performance of the instrument is negatively affected. Putting the final machined instrument through a finished instrument heat treat in a temperature range of about 248 to 500 C. (478 to 932 F.) restores and can enhance the fatigue-resistant properties of the instrument, with heat treat in the range of 350 to 450 C. (662 to 842 F.) being best. A longer time is required for heat treat at the lower end of this temperature range and a shorter time is required for heat treat at the higher end.
(26) Producing an instrument according to the process of FIG. 8 reduces the restoring force by about half when compared to the process of FIG. 8. Subjecting the finished instrument to a final instrument heat treat produces even less restoring force but with improved resistance to cyclic fatigue with minimal or insignificant loss in torque performance.
(27) An austenitic finish temperature A.sub.F above 37 C. is important for instruments adapted for use in the human body than is the stabilized martensitic effect and shape memory achieved by the prior art process of FIG. 4. Additionally, an A.sub.F above 37 C. may be achieved by eliminating the strain-induced thermal cycling steps of FIG. 4 and simply subjecting the Nitinol to a strain-induced final straightening thermal heat treat in the range of 410 to 440 C., varying the nickel percentage at the ingot level, or applying both strain-induced heat treat and varying the nickel percentage. This results in a Nitinol alloy with significant resistance to cyclic fatigue but without requiring the additional processing steps and elevated temperatures of prior art processes. Further, the alloy has a high A.sub.F. To date, no manufacturer of endodontic instruments produces an instrument in which the Nitinol is in a martensitic or transitory state (i.e. in the deformed monoclinic state) and having a high A.sub.F.
(28) While preferred embodiments of a Nitinol instrument made according to this invention have been described in enough detail for those of ordinary skill in the art, changes can be made to it without departing from the scope of this disclosure. Therefore, the present invention is only limited by the following claims, including equivalents to the individually recited requirements of each claim.
